In medical internal dosimetry, assessments of radiation dose to the active bone marrow in patients undergoing radionuclide therapy require separate assessments of the amount of radionuclide decays occurring within skeletal tissues, and the dose per radionuclide decay in the skeleton. Dosimetry models for Reference Male (70 kg in total mass and 170 cm in height) are currently utilized in assigning radionuclides for dose assessment, and thus some form of scaling is required. In this study, detailed in-vivo CT imaging, ex-vivo CT imaging, and high-resolution NMR microscopy of a variety of skeletal sites will be used to validate methods of scaling for improved patient-specificity of the active marrow dose estimate. Specific aims include: (1) Five male and five female cadavers will be selected in the age range of 55-70 years and with body mass indices in the range of 20-25 kg m-2. For each cadaver, multi-slice CT imaging will be performed in the head, torso, abdomen, and pelvic regions. These images will be used to assess skeletal spongiosa volumes in Specific Aim 4. (2) For each cadaver, multiple skeletal sites will be harvested and subjected to additional ex-vivo CT imaging at higher resolution. These images will be used to (a) verify in-vivo CT volumes of Specific Aim 1, (b) separate cortical bone and trabecular spongiosa, (c) segment trabeculae and marrow cavity volumes, and (d) determine sites of bone cube harvesting for NMR microscopy. (3) For each bone cube cut within Specific Aim 2, NMR microscopy will be performed. The resulting 3D images will be coupled to the EGSnrc transport code for simulation of electron and beta-particle emitters localized in the active marrow, endosteal bone surfaces, or bone trabeculae. (4) Finally, the dosimetry results of Specific Aim 3 will be used to test and verify a variety of scaling methods to include anthropometric parameters (total body mass, body height, lean body mass, body surface area, and body mass index), tissue volumes (active marrow and spongiosa), and tissue masses (active marrow and spongiosa). In addition, the data from Specific Aims 1 and 2 will be use to test the hypothesis that ratios of skeletal site volumes in one skeletal site are equivalent to those in other sites of high active marrow content (e.g., lumbar vertebrae, and pelvis, etc.)